1. Field of the Invention
The present invention generally relates to processes for removing toxic, hazardous or otherwise undesirable volatile organic compounds from soils, clays, sediments, sludges and the like. More particularly, this invention is concerned with processes for removing such volatile organic compounds from soils, etc., without employing incineration conditions.
2. Description of the Prior Art
Until recent times, relatively little attention has been paid to the public health consequences of introducing industrial wastes such as halogenated organic compounds into the environment. However, as man's understanding of the hazards associated with such compounds has grown, a body of stringent federal regulations has evolved to regulate their disposition. Such regulations also mandate the removal of these compounds, down to certain maximum permissible residual levels, from the soils of former disposal sites.
Cleanup operations of these sites are, however, very costly as well as technically difficult. They often involve the transportation of large volumes of contaminated soils, sludges, sediments etc. to specially designated and/or designed disposal areas or to specially designed and very strictly regulated incineration facilities. Obviously, the transportation of large volumes of contaminated soils, especially over relatively long distances, is an extremely expensive undertaking. In fact, transportation costs usually constitute the most significant cost factor in cleanup operations of this type. This cost factor is very frequently exacerbated by the fact that equally large volumes of uncontaminated soil must be brought from distant locations to replace the contaminated soil excavated from the cleanup site. Furthermore, the EPA has announced a five year goal of severely limiting and/or closing most hazardous waste landfills. The EPA has also announced its intention to more stringently regulate incineration of soils contaminated by organic chemicals in general and halogenated organic compounds in particular. Hence, the coming generation of incineration facilities will tend to become more technically complex and/or much more expensive to build and operate.
In response to these considerations, smaller, even portable, incineration systems have been proposed. For example, U.S. Pat. No. 4,667,609 discloses a mobile apparatus for infrared heating of soils contaminated by various hydrocarbons. The apparatus employed is provided with rotary seals to enable a slight negative pressure to be maintained in its furnace in order to prevent gaseous contaminants from leaking out to the atmosphere. Its heating step is however carried out to the point of complete combustion. Hence, its operation may be governed by certain federal regulations concerned with incineration of such materials. Similarly, U.S. Pat. No. 4,700,638 teaches a process wherein PCB's, dioxins, etc. are volatilized from soil at 350.degree.-500.degree. C. in order to avoid formation of glaze from certain low-melting soil constituents. Gases and mineral dust are removed by gas flow created by a fan and the resulting gases are combusted at high temperatures which may likewise invoke application of "incineration" regulations.
These regulations have their genesis in the fact that in many cases, small scale, local incineration--in spite of the best intentions and precautions--may well lead to air pollution problems which are even more pernicious than the original soil contamination problem being addressed. Not the least of these is the fact that incineration of certain halogenated organic chemical contaminants in the presence of water, oxygen and/or fixed nitrogen compounds can lead to the formation of various compounds (e.g., furans) which are known to be extremely toxic. Consequently, a number of alternative cleanup methods have been proposed which are intended to minimize transport costs and to completely eschew "incineration", as that term is defined by federal regulations, as a process step in any program for disposing of organic compounds in general and halogenated organic hydrocarbons in particular.
One such alternative to local incineration is embodied in the process taught by U.S. Pat. No. 4,574,013. It is based upon a concurrent reaction of an alkali metal hydroxide with an alcohol to form an alkoxide and water. The alkoxide is then reacted with a halogenated organic chemical contaminant to form an ether and a salt. The ether decomposes to a phenol which is then reacted with an alkoxide to form a soluble phenate. However, aside from the high cost of the chemical ingredients needed to carry out this process, as well as the complexities implied in its many process steps, this process is also greatly hampered by the fact that any water present in the feedstock will seriously interfere with the overall chemical reaction scheme. Consequently, the contaminated soil must first be thoroughly dried, at considerable expense before it undergoes this decontamination process.
U.S. Pat. No. 4,864,942 (the "942 patent") teaches a method for removing volatile organic compounds such as PCB's from soils by volatilizing said organic compounds at temperatures below those which federal regulations define as "incineration." This volatilization is induced by use of heat alone and is conducted in conjunction with continuous removal and condensation of any evolved vapors. The chief process vessel employed in process of the '942 patent is an indirectly heated rotary furnace which is operated at a very slight vacuum (i.e., a negative guage pressure from 0.1 to 2.0 inches of water column) in order to prevent vapor leakage into the atmosphere. This process has many virtues over others found in the prior art, but it also has one major drawback. The time periods necessary for it to effect complete volatilization of many high boiling point halogenated organic compounds are generally at least an hour. More often the process takes several hours, given the legal constraint to operate the device at relatively low temperatures less than 600.degree. F. Nonetheless, the '942 patent does disclose some technical insights which are also useful in describing, and in differentiating, applicant's invention; therefore, the 942 patent is incorporated by reference into applicant's patent disclosure.
For example, the 942 patent clearly notes that there are many "gaps" in man's knowledge of how liquids are released from solid inert materials such as soils. It notes that the "mechanism" by which complex materials (such as contaminated soils) are dried--so that substantially all liquid contaminants are removed--is indeed complex and by no means completely understood. This reference also points out that the technological phenomena believed to be involved are generally those disclosed in certain other recognized references such as: Paris, Physical Chemists Handbook, Section 21, which also is incorporated by reference into this patent disclosure. For example, it has been postulated that factors such as the structure of the solids in a given inert material, the type of liquid contaminant(s), the presence or absence of other liquids in the inert material, the concentration of liquid contaminant(s), and the saturation of the gas phase may all, simultaneously, influence the mechanism by which internal liquids flow through, and vaporize from, a given inert material. Moreover, such liquid flow mechanisms can be further influenced by such factors as: (1) diffusion, (2) capillary flow, (3) flow caused by shrinkage and pressure gradients, (4) flow caused by gravity and (5) flow caused by the applicable vaporization, condensation, sequence.
This all goes to say that the process of removing even one liquid chemical species from an inert, solid material is complex and rarely occurs as a single continuous process. Processes for removing mixtures of different chemical species are even more complex. In most cases all such processes will involve a number of distinct, stage-wise, phases. For example, a first phase in drying a soil, sludge, sediment, etc. (hereinafter referred to as "earth material(s)") generally will involve evaporation of liquids, which may be contaminants, water, or other liquids, from the saturated surface on the earth material. This is usually followed by a period of evaporation from a saturated surface of gradually decreasing area and, finally, when the surface of the earth material is no longer saturated, transition to a period of evaporation from the interior of a solid form of the earth material.
Hence, drying rates also will vary with the relative volatility of the contaminant(s), temperature, time, solids composition, and moisture content of the earth material being dried. The 942 patent also notes that a plot comparing vapor evolution versus time, will often indicate several distinct phases. For example, there is usually a first phase of gradually increasing evolution of vapors as a contaminated solid material warms up. A second phase, known as the constant-rate phase, corresponds to the period in which a constant amount of vapor is evolved. The constant-rate phase, in turn, continues until a point at which the rate of drying begins to fall. This is often referred to as the point at which a "critical-moisture" content point is reached. After reaching this critical-moisture content point, the next phase is often called the "falling-rate" phase. This phase is typified by a continuously changing rate throughout the remainder of the drying cycle which corresponds to a decrease in saturated surface area. The next distinct point in such a curve occurs at that point at which all the exposed surfaces become completely unsaturated. This marks the start of the portion of the drying cycle during which the rate of internal moisture movement controls the drying rate.
Again, the drying rate also depends on certain factors affecting the diffusion of moisture away from the evaporating surface and those affecting the rate of internal moisture movement. Moisture which is held in the interstices of solids, or held as a liquid on the surface, or is held as free moisture in cell cavities, moves by gravity and capillary flow, provided that passageways for continuous flow are present. Moisture may move by vapor diffusion through the solid material, provided that a temperature gradient is established by heating, to create a vapor pressure gradient. Vaporization and vapor diffusion may occur in any such solid material in which heating takes place from one direction, drying from the other, and in which liquid is isolated between or inside granules of solid.
In the terminal phase, during which mass transfer takes place primarily by molecular, rather than eddy diffusion, the drying rate is mostly governed by the rate of internal moisture movement as the influence of external variables diminishes. This period usually predominates in determining the overall drying time to some lower moisture content. Moreover, in those processes (such as the one disclosed in the 942 patent) which are carried out as a "continuous process", it should be noted that all of the above processes are occurring at the same time. It also should be specifically noted that the 942 patent does not teach or suggest that--at the high vacuum levels needed to quickly vaporize relatively nonvolatile contaminants (e.g., PCB's) to levels at or below certain maximum permissible residual concentrations (e.g., 2 ppm for PCB's)--there exist certain discontinuities in vaporization which can be utilized to expedite decontamination processes conducted at temperatures lower than those which legally defined "incineration."
It also should be noted that the 942 patent notes that an inert gas such as nitrogen, carbon dioxide, argon, etc., may be passed through such drying materials in order to reduce the partial pressure of the evolved vapors. In this way, the concentration of evolved vapors in the gas phase around the drying solid can be lowered, thereby making it easier for the heated liquids to pass from the liquid phase into the bulk of the vapor phase. Gases serving this purpose are often referred to as "sweep" or "sweeping" or "stripping" gases.
Finally, it should also be noted that the prior art has long recognized that as water residing in the interstitial spaces of a solid material vaporizes and goes into the vapor phase, it may carry certain contaminants along with it or otherwise facilitate the vaporization of such contaminants, e.g., by conditioning the gas phase to lower the vapor pressure at which the contaminants will pass into the vapor phase. It is also well known that even though the largest portion of water present in the inert solid vaporizes at around the saturation temperature of water, some water nevertheless goes into the vapor phase together with certain low boiling organics, and that residual water may still remain to be vaporized even in an inert solid which has been heated to a temperature above the boiling point of water. Hence, water may well play a significant role in increasing the effectiveness of a given decontamination process throughout a very broad range of temperature and pressure conditions which lie beyond the nominal boiling point of water at the overall total pressure prevailing.